1. Field of the Invention
The present invention relates to a controller having a function for changing a motion of a machine tool, depending on the temperature of a motor for driving a spindle of the machine tool, and a method for controlling the machine tool.
2. Description of the Related Art
In a machine tool having a spindle or a feed shaft driven by a motor, the temperature of a motor for driving the spindle increases, and therefore the motor may be overheated, when the machine tool carries out a heavy-duty cutting or a processing wherein the spindle is frequently accelerated or decelerated, etc. In order to avoid such a problem, for example, JP 2003-009563 A discloses a technique for detecting the temperature of a servomotor for driving a movable body, and controlling the servomotor so that an acceleration/deceleration time constant of the movable body is changed depending on the detected temperature.
JP H09-179623 A discloses a technique for generating temperature data by predicting and calculating the temperature of a feed shaft motor, comparing the temperature data to previously stored temperature data, and changing an acceleration/deceleration time constant of the feed shaft depending on the comparison result.
Further, JP 2009-041130 A discloses a technique for calculating a virtual motor temperature based on an average load torque of a motor for driving a carriage of a flat-knitting machine, and reducing the voltage applied to the motor when the virtual temperature exceeds an admissible value.
In the technique of JP 2003-009563 A or JP H09-179623 A, the acceleration/deceleration time constant is changed based on the detected or estimated motor temperature. Generally, when a certain torque is necessary to accelerate or decelerate the motor and the motor should be frequently accelerated or decelerated, an amount of heat generation of the motor is increased. Therefore, it is effective to change (extend) the time constant in order to decrease the amount of heat generation. However, the motor may be overheated not only when the motor is accelerated or decelerated, but also when a heavy-loaded processing is continuously carried out. It is not effective for the latter case to extend the time constant.
On the other hand, the technique described in JP 2009-041130 A may be effective for the flat knitting machine as disclosed, i.e., wherein the rotation torque of the motor is generally constant even when the velocity is varied. However, this technique cannot be applied to a machine tool wherein the cutting load is rapidly increased when the motor velocity is decreased.
As described above, as factors in overheating of the spindle of the machine tool, high current due to the acceleration/deceleration (and the frequency thereof), and high current due to heavy cutting load (and a duration time thereof) may be possible. In the prior art, however, these factors cannot be discriminated from each other, and therefore it is difficult to take appropriate measures depending on the factors.